In preserving fresh foods such as cakes, meat, vegetables, and fruits at low-temperature in a case appropriate humidity is required inside the case so that the air therein is not only too dry but also, in the case of preservation of cakes in particular, not too humid. In view of these points there has been proposed a low-temperature foods preserving case having two evaporators and a fan in the passage of the cooling air, and is capable of maintaining high humidity inside the case by operating the evaporators alternately by means of a timer in such a way that, while one of the evaporator is in operation, the other evaporator is stopped to permit the defrosted water collected therefrom into a reservoir to evaporate (Japanese Utility Publication 62-9511).
However, since the above conventional case has two evaporators arranged at up and down bi-levels in the case, so that, after being circulated half way round the air route of the air circulated by the fan in the case, the humidity contained in the air tends to condense and gets deposited on the cooling evaporator which is evaporating the refrigerant. Consequently, the cooling air passed through the evaporator loses its humidity and has a poor humidification in the later half of the air circulation route.
Further, since the defrosted water is left in the reservoir for natural evaporation, the surface area of the water in contact with the air is small, and since the temperature of the water is too low for good evaporation, the the natural evaporation is not great and difficult to maintain high humidity inside the case as required.
There is a proposition on the use of an electric heater for heating the two evaporators for facilitating good evaporation of the water deposited on one of the evaporators which is stopped for defrosting operation in the alternate stopping and running cycle controlled by a timer provided in the case. See, for example, Japanese Patent Application Laid Open 62-9670.
This prior art heating is controlled by the timer for turns ON the heater and by a temperature detector which turns OFF the heater as the temperature reaches a predetermined temperature. Incidentally, the defrosting period of time during which one of the two evaporators is stopped for defrosting operation under said timer control, is obviously set longer than the heating period.
Therefore, the conventional defrosting operation of the evaporator is carried out intensively over a short time interval in the first half of the predetermined defrosting period started by the timer. During this period most of the water deposited on the defrosting evaporator is evaporated, but is again condensed on the cooling evaporator. The evaporation of the water from the defrosting evaporator drops sharply thereafter. Thus, the conventional system has a disadvantage that the water is not evaporated evenly during the defrosting period, so that the humidification inside the case by means of the defrosted water is not obtained as desired.
Also in such a conventional case, temperature sensors provided at various positions each have their own definite roles, so that a temperature control sensor that has failed to function cannot be backed up by other sensors, resulting in the malfunction of the temperature control system as a whole.
In order to maintain a preferable temperature inside such a conventional case even in the event of the malfunction of the temperature control system, a ON-OFF switching of the cooling apparatus by means of a timer has been proposed, as described in Japanese Utility Publication 62-16601.
Hoverer, this conventional system requires tedious adjustment of the duty cycle of the temperature control system so as to meet the environmental conditions every time the system has failed, since the setting of ON and OFF duty periods is not automatically adjusted.